Triplex propulsion system and method



Filed Nov. 3, 1960 R. M. BRIDGFORTH, JR

TRIPLEX PROPULSION SYSTEM AND METHOD M Mm .M W m W f, m wx m WM f afmDec. 3, 1963 /oo WH/4057' M2225 Aff/4 164 770 United States Patent O3,112,608 TRIPLEX PROPULSEON SYSTEM AND METHGD Robert M. Bridgforth,Jr., Mercer Island, Wash., assignor to Rocket Research Corp., Seattle,Wash., a corporation of Washington Filed Nov. 3, 1960, Ser. No. 67,017

` 7 Claims. (Cl. Gli-35.4)

and to decrease the average molecular weight as well as increase thevelocity Yof the thrust developing reaction stream. In particular, anembodiment of such a system can employ a lithium or lithium containingfuel component, iluorine as the oxidizing component, and hydrogen as`the Working fluid, which constituents provide what may be termed atriplex propellant.

The unit of measure of the power of a propellant relamenti@ specicimpulse, defined as the thrust of the rocket engine.` divided by thellow rate of the propellant. Until the present time, it has been theopinion of those concerned with rocket propellants that the systemhydrogen-fluorine gave the highest specific impulse of any ystablechemical rocket propellant. This invention discloses a stable chemicalrocket propellant with specific impulse substantially greater than thatof the hydrogen- Vfluorine system. V

Previously, in the selection of rocket propellants, attention has beendirected toward those substances which, when reacted together, producehigh temperature exhaust gases. These gases, the reaction products ofthe 4fuel and the oxidizer, are then expanded through a nozzle Y toleave the rocket at high velocity and exert a thrust upon `the rocket.

In my invention the functions of producing heat and l of Yconvertingthermal energy to kinetic energy are cony, sidered separately. Twoclasses of chemicals are used.

One class is called the reactants, and these chemicals involve afuel-oxidizer' pair, selected primarily for their ability to producelarge quantities of heat per unit mass. For purposes ofthe presentinvention, lithium has been ,found to be the best fuel component Vanduorine the .best oxidizer component. The second class is called `therworking lluid, and is selected primarily for the ability to .receivethe heat produced by the reactant and to'conyvert this Vthermal energyinto kinetic energy. Forpur- -poses of the present invention, hydrogenhas been found 4to be the most effective working fluid.

, In particular, the reaction Asystem of the present invenytioncomprises a triplex system involving lthium-fluorinehydrogen, whichcomponents in their intermixture, chemical reaction, and lthermalinteraction as a rocket propel- Ylant in a rocket engine produce ahigher speciic impulse than either a hydrogen-fluorine system, or alithium fluorine system. As indicated, lithium and uorine are employedas the reactants, and hydrogen is employed as the Working fluid. Whenlithium reacts with fluorine, lithium fluoride is formed. This substancehas a normal 4boiling point of-2,0t80fK. The standard heat of reactionof lithium with fluorine is 5,385 B.t.u./lb. if the lithium fluoridereaction product is in the vapor state. However, if the lithium fluoridereaction product is transformed to a' liquid or solid state, thestandard heats of reaction are respectively '9,856 B.t.u./lb. or 10,-150B.t.u./

of the reaction and discharge of the reaction stream, giving asubstantially higher heat of reaction (i.e. available heat), andproviding a gaseous phase primarily constituted by loW molecular weighthydrogen (M.W. 2) rather than the high molecular weight lithium fluoride(M.W. I25.9). d

In its broader aspects, the present invention compre hends utilizationof an essentially non-reacted,.ve'ry low molecular weight working iluidwith a reaction product which is normally gaseous in the absence ofa'working uid but which is condensable at a relatively high temperature,i.e. condensable at an elevated temperature compatible with customary,art-recognized reaction engine exhaust gas temperature requirements. A YV If no working 4fluid is mixed with the lithium-iluoride reaction, thereaction chamber temperature is about 5,570 K., and the temperature `ofthe discharge stream emerging from the nozzle is about 3,640" K.,assuming a stoichiometric reaction and `operating conditions as denedwith respect to accompanying FIG. 1. It is important to note withrespect to `such operation 'without a working fluid that .thetemperature =of the discharge stream at the exit plane is substantiallyhigher .than the normal boiling point of lithium fluor-ide, 2,080o K.,so that the lithium fluoride reaction product remains in the vapor stateuntil `well past the exit plane of the discharge nozzle. However, if aworking fluid, such asxhydrogen in the amount of 20% for example, isinjected into the reaction chamber, under the same conditions, thetemperature in the `reaction chamber is reduced to about (2,720" K., andthe temperature of the discharging stream at the exit plane is about1,640 K., which is Well below the normal boiling point of LiF (2,080K.), and which results in a predominant part lof the reaction productbeing in non-gaseous phase sat the exit plane. In actuality not all ofthe .reatcion product is condensed in .any event because off theexistence of partial pressure equilibria. `Manifestly,V

the extent of cooling incurred by the working fluid can be varied asdesired to achieve a particular-manner of operation, depending upon therelative proportion of working fluid introduced to the reaction. Theimportant consideration is that `suiiicient working fluid be added tothe reaction to accomplish an exhaust temperature below about 2,080? K.at theexit plane, i.e. to accomplish condensation of the expandingdischarge stream of a large .part of the condensable reaction product.

Although hydrogen may possibly enter into Vchemical combination withfluorine to a small extent in -the combustion chamber, at thetemperatures existing in the exhaust flame there is .no -appreciablestable reaction 'between hydrogen yand 'lithium fluoride, andtheprincipal chem-ical species present in .the exhausting products aregaseous hydrogen and condensed lithium lluoride, in either liquid orsolid forni. The high heat of formation of lithium fluoride, coupledwith the low molecular weight of hydrogen, results in a system having averyY high specific impulse. The mode -by which this invention operatescan be explained by refer-ring to the equation for Vspecific impulse.

When the exit pressure equals ambient' pressure, the

specific impulse of a following equation:

propulsion reactionv is Vigiv'errby the Pe=exit pressure R=gas constantl+mass flow rate of condensed phase mass low rate of gas phase M :meanmolecular weight of gases.

Z. mass flow rate of Working fluid added to reactants total mass iowrate through rocket and since To is much less than Tc, the aboveequation for specific impulse becomes, approximately,

By examining the above equation, the effect of adding hydrogen as aworking fluid to a lithium-fluorine reactant system can be seen. As thehydrogen content increases the following eifects occur:

(1) The factor (1e-Z) decreases, tending to decrease the specificimpulse.

(2) More and more lithium uoride is condensed and the high heat ofcondensation of lithium fluoride thus causes Qc to increase, tending toincrease specic impulse.

(3) As lithium fluo-ride condenses, ,B increases, tending to decreasespecific impulse until eventually essentially all of the lithium uorideis condensed, and adding more hydrogen will again decrease (4) Theaddition `of low molecular Weight hydrogen causes the mean molecularweight M of the gas to decrease land condensation of .the lithiumfluoride remo-ves it from the gas phase and eventually M decreases toapproximately 2.016, with the eiect of lincreasing specific impulse.

(5) Due to the high specific heat of hydrogen, lthe mean specific heatCp increases, tending 'to decrease specific impulse.

The composite result of all these effects is that as hydrogen is addedto lithium-fluorine, the specic impulse first increases, and thendecreases when the hydro-gen content is increased above about 30% byweight.

The specific impulse values of the system lithiumliuorine-hydrogen, withno losses and with shifting equilibria, are shown in the following TableI:

Table I [Specific impulse at sea level-Lithum-fluorine-hydrogen. Chamberpressure=1000 p.s.i.a.; exit pressure=14-7 p.s.i.a.]

Specific (Wt. Fluorine)/(Wt. Lithium) Wt. Percent Impulse Hydrogen lb.f.scc./

lb. m.

2.74 (stoiehiometric) O 389 2.74 396 15 409 422 432 436 434 426 401 0302 20 398 30 412 50 374 0 163 10 407 30 392 30.0 50 355 In thefollowing Table II, the vacuum specic impulse is shown, defining theperformance of the propellant in the environment of outer space:

Table II [Vacuum specific impulse-Lithlurn-flourine-hydrogen` Chamberpressure-1000 p.s.i.a.; (Wt. Iiuorine)/(wt. lithium)2.74. Hydrogenconcentration selected for maximum specific impulse] Exhaust Nozzle AreaRatio The following Table III shows the gains in specific impulse whichthe lithium-fluorine-hydrogen system produces over the hydrogen-oxygensystem and the hydrogenliuorine system:

T able III.-Lthium-Fluorne-Hydrogen [Specific impulse expressed in lb.f.-see./lb.m. Hydrogen concentration selected for maximum specificimpulse] The maximum sea level specific impulse is obtained with thefollowing composition:

Wt. percent hydrogen=30 (Wt. uorine) (wt. hydrogen) :2.74

As shown in Table 1, this propellant composition gives a sea-levelspecific impulse of 436 lb. f.sec./ lb. m., which is 27 lb. f.sec./lb.m. greater than that of hydrogenuorine, the previously supposed mostpowerful stable chemical propellant, having under the same conditions aspecic impulse of 409 lb. f.-sec./1b. m. It is also to be noted that thesea-level specific impulse of the lithiumuorine-hydrogen system is 42lb. f.sec./lb. m. greater than that of the system hydrogen-oxygen, acurrently popular high energy propellant which has under the sameconditions a specific impulse of 394 lb. f.sec./lb. m. With large arearatio exhaust nozzles, the gains in vacuum specific impulse overconventional systems become greater, approaching up to 138 lb.f.sec./1b. m. over hydrogenoxygen and approaching up to 171 lb.f.sec./lb. m. over hydrogen-Huorine as the exhaust nozzle area ratiobecomes indefinitely larger. As the exhaust nozzle area ratio becomeslarger, the composition which produces maximum vacuum specific impulsecorresponds to a stoichiometric ratio of fluorine to lithium, (Wt.uorine) (wt. lithium)=2.74, but the concentration of hydrogen formaximum vacuum specific impulse varies from a Weight percent of 30 at anozzle area ratio of 30 to a weight percent of 20 at a nozzle area ratioof 10,000 to a weight percent of zero at an infinite area ratio.

For purposes of ready comparison, and to show the substantially improvedperformance characteristics of the tri-propellant system here presented,accompanying FIG. 1 graphically shows the variations of sea-levelspecific impulses of hydrogen-fluorine and hydrogen-oxygen systems aswell as the lithium-fluorine-hydrogen system 0f the present invention,assuming a reaction chamber pressure of 1000 p.s.i.a. and an atmosphericexit pressure; FIG. 2 shows a corresponding comparison of the vacuumspeciiic impulses of the three systems under a typical exhaust nozzlearea ratio of 50; and FIG. 3 similarly compares the maximum vacuumspecific impulses of the three systems at various exhaust nozzle ratios.

" `the specific impulse.

equipment required to handle a third component. Ac- Y cordingly, thehydrogen content should preferably be between about 20% and about 40% byweight. When the hydrogen content is raised above about 50% by Weight,the over-all propellant density becomes relatively low, requiring heavytankage, and because of the large diluting effect of the hydrogen, thespecific impulse decreases below that of a hydrogen-oxygen system.

For stoichiometric reaction between lithium and uorine to form lithiumfluoride, the ratio (wt. iiuorine)/(Wt. lithium) should be about 2.74.When excess lithium is added to the system, reducing the fluorine tolithium ratio below 2.74, the excess lithium is essentially chemicallyinert in this environment, and the excess serves to reduce the heat ofreaction per unit mass of propellant, reducing In addition, the heat ofvaporiza- -tion of lithium is Very high, being 32.48 K. cal/mole, and

this contributes to additional degradation in performance.

Finally, lithium vapor, with an atomic weight of 6.94, is

added to the gas phase, increasing the average molecular weight of thegas and tending to reduce performance.

The addition of excess iuorine, above the stoichiometric ratio, producesappreciable quantities of hydrogen fluoride, which is entirely gaseous,reducing the Weight percent of the condensed phase, and increasing theoverall propellant density, but reducing the over-all heat of reaction.With -a very large fluorine content, the system approaches` thehydrogen-fluorine system, and the presence of small amounts of lithiumis unjustitied because of the small performance gain produced and theincrease in complexity required to introduce a third component.Therefore, the ratio-(wt. fluorine)/.(wt. lithium) should usually beclose to 2.74, andthe entire working range runs lfrom about 2.00 toabout 30.0.

The optimum constituency of the propellant of the present inventiondepends upon the speciiic mission which the rocket system is tokperform,and does not always exactly correspond to the composition giving maximumspecic impulse. Tank weights must-be considered, in a study of theparticular mission and the propellant requirements. For some missions, acomposition which has a greater over-all propellant `density may havelower tank weights and may give better over-all vehicle perfomance thangiven by the exact constituency having maximum specific impulse. Theoptimum propellant composition, with the optimum compromise between highspeciiic impulse and high propellant density, therefore depends upon thespecific mission.

This propellant system is intended to be utilized in a rocket engine bytechniques generally known per se in lrocket technology. Storagechambers .are provided for the propellants, which are suitablytransported from the storage chambers to the reaction chamber. Here,mixing and chemical reaction take place, the lithium reacting with theiluorine to form lithium fluoride, and releasing large quantities ofheat. To `a lesser degree, the hydrogen reacts with the iiuorine to formhydrogen fluoride, but the *Y principal function of the hydrogen is tomix with the lithium fluoride, to receive its heat, and to expandthrough a converging-divergin-g nozzle, carrying along the condensedlithium uoride, and leaving the rocket at high vvelocity, exerting athrust on the rocket.

The hydrogen and the fluorine can be carried in propellant tanks asliquids, using the technology of handling cryogenic iiuids which is nowwell developed -by the rocket industry. In regard to the lithium, thisfuel may be supplied as a solid or as a liquid. Lithium has a meltingpoint `of 186 C., and can be melted in the vpre-launch operations orduring flight; In regard to lits utilization in solid form, the lithiummay be powdered and introduced into the reaction chamber as a powder, orit maybe introduced in the form yof wire. Alternately, the lithium maybe carried in the reaction cham-ber in suitable solid form, such as achamber liner, and can con-stituently be either substantially pure orcontaining relatively small quantities of lithium hydride, lithiumperchlorate, .or ammonium perchlorate, for example.

The lithium, fluorine, and hydrogen may be injected,

exposed or otherwiseintroduced to the reaction in the chamber atessentially the uniform rates and in such a manner as to obtain the mostrapid mixing of all three. A modiication of this method of injectionconsists in designing the injector so that the lithium'and the fluorineare mixed first, and the resulting lithium iluoride is then mixed withhydrogen, which has been injected around the walls at the primaryinjector station or which has been injected downstream of the primaryinjector station.

In another utilization layout for the system, V-a portion of thehydrogen is carried in the for-m of lithium hydride 'by having a portionor all of the lithium in the form of lithium hydride. In the followingTable .IV and Table V, it is seen that the specific impulse of thelithium hydridefluorine-hydrogen system is not as great as thelithiumiluorine-hydrogen system. However, the over-all density of thepropellants is increased and under some circumstances it may bepracticably desirable to have a system comprising lithium hydride,fluorine, and hydrogen;

Table IV [Speeie impulse at sea-level-Lithium hydride-fluorine-hydrogen.

hamber pressure=1000 p.s.i.a.;

exit pressure=14.7 p.s.i.a.; (wt. iiuorine)/(Wt. lithium)=2.74]

- Specic im- Wt. percent uncombined hydrogen pulse, 1b. f.-

sec./1b. m.

Table V Vacuum speciieimpulse-Lithium hydride-fluorine-hydrogen. Chamberpressure= 1000 p.s.i.a.; (Wt. uorine)/ (wt. lithium) =2.74. Hydrogenconcentration selected for maximum specific impulse] chamber for thethree systems hydrogen-fluorine, hydrogen-oxygen, andlithium-fluorine-hydrogen, all such sys-V tems being compared atproportions given maximum specific impulse at sea-level. From aconstruction standpoint and a material-usage standpoint, the lower theternperature of combustion the longer the life of the materials ofconstruction and, also, the lighter the Weight of the materials ofconstruction that can be used. Therefore, it is seen that thelithium-uorine-hydrogen system is superior to the other two systems inTable VI.

7 Table VI propellant 'l'empcellltlillrinu rneacton Hydrogen-ilumine6500 Hydrogen-oxygen 4930 Lithum-fluorine-hydrogen 3530 The speciicimpulse values reported in this specification are based upon thepropellants in the following initial states: liquid hydrogen at itsnormal boiling point ot 20 K.; liquid fluorine at its normal boilingpoint of 85 K.; liquid lithium at 500 K.; and solid lithium hydride at300 K.

From the foregoing specification it is seen that there is represented alithium-iluorine-hydrogen rocket propulsion system, which tripropellantsystem is characterized essentially by a chemical reaction between thelithium and fluorine releasing heat, and with hydrogen acting tocondense the lithium tluoride and increase the heat req leased from thelithium fluoride, and with the hydrogen acting as a low molecular weightgas to efficiently con` vert thermal energy into kinetic energy,producing a high specific impulse.

From the foregoing considerations, various further modifications,formulations, and utilization techniques characteristic of the inventionwill be apparent to those skilled in the art, within the scope of thefollowing claims.

What is claimed is:

1. A method of generating thrust in a rocket engine; said methodcomprising mixing lithium, iiuorine and essentially uncombined hydrogenin a reaction chamber, and discharging the lithium fluoride reactionproduct and admixed hydrogen through an exhaust nozzle at high velocity,producing thrust; the weight ratio of fluorine to lithium lying betweenabout 2 and about 30; and the weight percent of the hydrogen in thetotal mixture being between about l0 percent and about 50 percent.

2. A method of generating thrust in a rocket engine; said methodcomprising mixing lithium, lluorine and essentially uncombined hydrogenin a reaction chamber, and discharging the lithium iluoride reactionproduct and admixed hydrogen through an exhaust nozzle at high velocity,producing thrust; the weight ratio of liuorine to lithium beingapproximately 2.74, and the weight percent Cil.

of the hydrogen in the total mixture being between about 20 percent andabout 40 percent.

3. A method for generating thrust in a rocket engine; said methodcomprising mixing lithium hydride, lithium, iluorine and essentiallyuncombined hydrogen in a reaction chamber and discharging the lithiumfluoride reaction product and admixed hydrogen through an exhaust nozzleat high velocity, producing thrust; the weight ratio of iluorine tototal lithium being approximately 2.74, and the weight percent of thehydrogen in the total mixture being between about 15 percent and about30 percent.

4. A propellant composition comprising lithium fluoride and essentiallyuncombined hydrogen; the weight ratio of uorine to lithium lying betweenabout 2 and about 30; and the weight percent of the hydrogen in themixture being between about 20 percent and about 50 percent of the totalweight of the composition.

5. A propellant composition consisting essentially of lithium Iluorideand uncombined hydrogen, the weight percent of the hydrogen in the totalmixture being between about 20 percent and about 4-0 percent.

6. A propellant composition comprising lithium hydride, lithium, uorine,and essentially uncombined hydrogen; the weight ratio of uorine to totallithium being approximately 2.74, and the weight percent of theuncombined hydrogen in the total mixture being between about 15 percentand about 30 percent.

7. A composition of matter consisting essentially of at leastprincipally non-gaseous lithium uoride in admixture with essentiallyuncombined hydrogen, the Weight percent of the hydrogen in the totalmixture being between about 20 percent and about 50 percent.

References Cited in the le of this patent UNITED STATES PATENTS Zwickyet al Oct. 29, 1957 Rae Oct. 18, 1960 McGrew May 15, 1962 a 72, December1947, pp. 10-23.

1. A METHOD OF GENERATING THRUST IN A ROCKET ENGINE; SAID METHODCOMPRISING MIXING LITHIUM, FLUORINE AND ESSENTIALLY UNCOMBINED HYDROGENIN A REACTION CHAMBER, AND DISCHARGING THE LITHIUM FLUORIDE REACTIOPRODUCT AND ADMIXED HYDROGEN THROUGH AN EXHAUST NOZZLE AT HIGH VELOCITY,PRODUCING THRUST; THE WEIGHT RATION OF FLUORINE TO LITHIUM LYING BETWEENABOUT 2 AND ABOUT 30; AND THE WEIGHT PERCENT OF THE HYDROGEN IN THETOTAL MIXTURE BEING BETWEEN ABOUT 10 PERCENT AND ABOUT 50 PERCENT.
 7. ACOMPOSITIN OF MATTER CONSISTING ESSENTIALLY OF AT LEAST PRINCIPALLYNON-GASEOUS LITHIUM FLUORIDE IN ADMIXTURE WITH ESSENTIALLY UNCOMBINEDHYDROGEN, THE WEIGHT PERCENT OF THE HYDROGEN IN THE TOTAL MIXTURE BEINGBETWEEN ABOUT 20 PERCENT AND ABOUT 50 PERCENT.